Object Oriented Software Design (NTU, Class Even, Spring 2009) Final Examination Problem Sheet TIME: 06/16/2009, 14:20 17:20

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1 Final Examination Problem Sheet TIME: 06/16/2009, 14:20 17:20 This is a closed-book exam. Any form of cheating or lying will not be tolerated. Students can get zero scores and/or fail the class and/or be kicked out of school and/or receive other punishments for those kinds of misconducts. Both English and Traditional Chinese (if suited) are allowed for answering the questions. We do not accept any other languages. 1 POO Zoo The two unicorn friends (Pinky and Bluey) of Charlie the unicorn are persuading him to come to the POOZoo, the POOCasino, or the POOBoard, instead of the candy mountain. To add those popular characters to POO, one of the members in your programming team wrote the following code: 1 class Unicorn { 2 int x, y ; 3 static void walk ( Unicorn u ){ u. x += 2 ; } 5 class C h a r l i e { 6 private int count ; 7 S t r i n g r e j e c t ( S t r i n g p l a c e ) { return I don t want to go to + p l a c e ; } 8 S t r i n g agree ( S t r i n g p l a c e ){ return Okay, okay, I l l go to + p l a c e ; } 9 public void do unicorn walk ( ) { Unicorn. walk ( this ) ; } 10 public S t r i n g a c t i o n ( S t r i n g p l a c e ){ 11 do unicorn walk ( ) ; count++; 12 i f ( count < 2) return r e j e c t ( p l a c e ) ; 13 else { count = 0 ; return agree ( p l a c e ) ; } 1 15 } 16 class Pinky{ 17 public void do unicorn walk ( ) { Unicorn. walk ( this ) ; } 18 public S t r i n g a c t i o n ( S t r i n g p l a c e ){ 19 do unicorn walk ( ) ; 20 return Charlie, Charlie, l e t s go to + p l a c e ; 21 } 22 } 23 class Bluey { 24 public void do unicorn walk ( ) { Unicorn. walk ( this ) ; } 25 public S t r i n g a c t i o n ( S t r i n g p l a c e ){ 26 do unicorn walk ( ) ; 27 return Yes, Charlie, + p l a c e ; 28 } 29 } The other programmer in your team write the following code: 1 class Demo1{ 2 public static void main ( S t r i n g [ ] argv ){ 3 Unicorn [ ] uarr = new Unicorn [ 3 ] ; 4 uarr [ 2 ] = new C h a r l i e ( ) ; uarr [ 1 ] = new Bluey ( ) ; uarr [ 0 ] = new Pinky ( ) ; 5 for ( int t = 0 ; t < 2 ; t++) 6 for ( int i = 0 ; i < uarr. l ength ; i++) 7 System. out. p r i n t l n ( uarr [ i ]. a c t i o n ( the POO Zoo ) ) ; 8 } 9 } You want the two pieces of code, when coupled together, can show the correct output like Charlie, Charlie, let s go to the POO Zoo 1 of 5

2 Yes, Charlie, the POO Zoo I don t want to go to the POO Zoo Charlie, Charlie, let s go to the POO Zoo Yes, Charlie, the POO Zoo Okay, okay, I ll go to the POO Zoo However, the code above doesn t work, because it cannot pass the compiler (hahaha)! (1) (5%) Assume that you can ask another programmer to re-write the latter piece of code, while the former remains unchanged. Write down the correct and clean lines that you expect her/him to write to cope with the former piece. (2) (15%) Assume that you can ask another programmer to re-write the former piece of code, while the latter remains unchanged. Write down the correct and clean lines that you expect her/him to write to cope with the latter piece. Note that the shortness of your answers is a big concern, of course, because you are in a time-bounded exam. The quality of your answers would also be taken into account. 2 POO Calculus Yes, yes, yes. It is purely evil to remind you that your calculus exam is coming soon. But imagine that you can bring your well-designed POO Calculus code to the JVM of your cellphone to the calculus exam, wouldn t it be fantastic? Let s start developing this useful software for future students. One basic component in calculus is the real-valued function from R to R. We will define it as an abstract Java class to begin with. 1 abstract class Function { public abstract double e v a l ( double x ) ; } A (symbolically) differentiable function f(x) is a function such that f (x) has a closed form; a (symbolically) integrable function g(x) is a function such that g(x)dx has a closed form. The two properties would be represented by the following interfaces. 1 interface D i f f e r e n t i a b l e { Function d i f f ( ) ; } 2 interface I n t e g r a b l e { Function i n t e ( ) ; } Thus, a function h(x) = exp( 0.5 x 2 ) would probably be like 1 class SDExp extends Function implements D i f f e r e n t i a b l e { 2 public double e v a l ( double x ){ return Math. exp ( 0.5 x x ) ; } 3 public Function d i f f ( ) { return new Multiply (new Poly ( 1.0, 1 ), this ) ; } (1) (5%) We know that the exponential function g(x) = exp(x) is both differentiable and integrable, with g (x) = g(x) = g(x)dx. Complete the code below. 1 class Exp extends Function implements D i f f e r e n t i a b l e, I n t e g r a b l e { 2 // w r i t e your code 3 } (2) (5%) We know that the simple polynomial function g(x) = αx D is both differentiable and integrable, with g (x) = αdx D 1 and g(x)dx = α D+1 xd+1. Complete the code below. 1 class Poly extends Function implements D i f f e r e n t i a b l e, I n t e g r a b l e { 2 double deg ; 3 double c o e f ; 4 Poly ( double coef, double degree ){ this. c o e f = c o e f ; this. deg = deg ; } 5 // w r i t e your code 6 } 2 of 5

3 (3) (5%) The Sum class represents the sum of two functions. Use the fact that (f + g) = f + g and (f + g) = f + g to complete the code below. What would happen if either f or g is not symbolically differentiable? 1 class Sum extends Function implements D i f f e r e n t i a b l e, I n t e g r a b l e { 2 Function f, g ; 3 Sum( Function f, Function g ){ this. f = f ; this. g = g ; } 4 // w r i t e your code 5 } (4) (5%) Without simplifying any equations manually, write the code that computes the value of d ( 3x 2 + 2x ( exp(x)dx) ). dx x=2 Note: You can use Math.exp(double a), which returns exp(a).you can also use Math.pow(double a, double b), which returns a b. 3 POO Search POO is deciding to make many things searchable boards, articles, friend lists, messages, to name a few. Of course, one wouldn t expect to waste time on writing the same search program several times. Hence, Java Generics is the time-saver (?!). Your programming team decides to start by having a Searchable interface, where its match function determines whether the instance matches the query string. 1 interface Searchable { boolean match ( S t r i n g query ) ; } 2 class POOBoard implements Searchable { /... / } 3 class POOFriend implements Searchable { /... / } 4 class POOGoodFriend extends POOFriend{ /... / } Now, the team wants a Searcher class that works with anything Searchable. A naïve idea is 1 class Searcher extends ArrayList<Searchable >{ /... / } However, such a Searcher allows us to mix POOBoard and POOFriend altogether in the list, which does not look very right. What the team really wants is something like 1 class Searcher <E> extends ArrayList<E>{ /... / } But that doesn t look right, either, because we can then have something like 1 Searcher<Integer > s = new Searcher <Integer >(); 2 s. add (new I n t e g e r ( 3 ) ) ; Nevertheless, Integer does not implement Searchable and hence should not be allowed by Searcher. (1) (10%) Write down the declaration of Searcher that would allow it to work on only any Searchable type but not a mixture of them. Your declaration should look like 1 class Searcher<E > extends ArrayList<E > where the underlines should be replaced by your own code or removed. (2) (5%) The core method of Searcher is a method called search, which looks like 1 Searcher <E > search ( S t r i n g query ){ 2 Searcher<E > r e s u l t = new Searcher<E >(); 3 for ( int i =0; i<this. s i z e ( ) ; i ++){ 4 E o = this. get ( i ) ; 5 i f ( o. match ( query ) ) { r e s u l t. add ( o ) ; } 6 } 7 return r e s u l t ; 8 } 3 of 5

4 Fill in the necessary part in the underlines above that would allow the code below to work. 1 Searcher <POOBoard> s = new Searcher<POOBoard>(); 2 // add some POOBoards t o s 3 Searcher <POOBoard> r e s = s. search ( Gossip ) ; (3) (5%) Another method of Searcher is called addsearchresult, which looks like 1 Searcher< > addsearchresult ( Searcher< > orig, S t r i n g query ){ 2 this. addall ( o r i g. s earch ( query ) ) ; 3 return this ; Fill in the necessary part in the underlines above that would allow the code below to work. 1 Searcher <POOGoodFriend> sg = new Searcher<POOGoodFriend >(); 2 // add some POOGoodFriends t o s g 3 Searcher <POOFriend> s2 = new Searcher<POOFriend >(); 4 Searcher <POOFriend> r2 = s2. addsearchresult ( sg, CharlieL ) ; 4 POO Exception Consider the following two exceptions that are used in POO. 1 class NetworkDisconnectException extends IOException { } 2 class UserNotExistException extends Exception { } The following hierarchy in Java might also be helpful. 1 class Exception extends Throwable{ } 2 class IOException extends Exception { } 3 class RuntimeException extends Exception { } 4 class NullPointerException extends RuntimeException { } (1) (20%, with partial credit) A lazy programmer in your team wrote the following exception handling code, which, of course, is not of the top quality. Please use the correct try-catch-finally mechanism to make the code better-organized while doing exactly the same thing (if exception only happens during action 1, action 2, and action 3). 1 try{ 2 a c t i o n 1 ( ) ; 3 i f ( a c t i o n 2 ( ) ) { a c t i o n 3 ( ) ; } 4 else { a c t i o n 0 ( ) ; return ; } 5 } 6 catch ( Throwable e ){ 7 a c t i o n 4 ( ) ; 8 i f ( e instanceof NullPointerException ){ a c t i o n 5 ( ) ; } 9 i f ( e instanceof Exception ){ a c t i o n 6 ( ) ; } 10 i f ( e instanceof NetworkDisconnectException ){ a c t i o n 7 ( ) ; } 11 i f ( e instanceof UserNotExistException ){ a c t i o n 8 ( ) ; } 12 a c t i o n 0 ( ) ; 13 } Note that the shortness of your answers is a big concern, of course, because you are in a time-bounded exam. The quality of your answers would also be taken into account. 4 of 5

5 5 POO Love It is nearly the Lunar Valentine s Day, and POO BBS decides to have an activity of love sending. Basically, any user is allowed to send love messages to as well as to receive love messages from other users. To handle the process, every user is modeled as an instance of POOUser that contains an instance variable love count, which is increased by 1 whenever a love message is received. A POOLoveThread is associated with each user to handle the sending process. 1 class POOUser{ 2 private int l o v e c o u n t ; 3 public void i n c r e a s e l o v e c o u n t ( ) { int r = l o v e c o u n t + 1 ; l o v e c o u n t = r ; } 5 6 c l a s s POOLoveThread extends Thread{ 7 POOUser user ; 8 POOLoveThread (POOUser user ){ this. user = user ; } 9 void i n c r e a s e l o v e c o u n t (POOUser r e c e i v e r ){ 10 r e c e i v e r. i n c r e a s e l o v e c o u n t ( ) ; 11 } 12 public void run ( ) { 13 while ( true ){ 14 POOUser r e c e i v e r = f i n d u s e r ( a s k r e c e i v e r ( user ) ) ; 15 i n c r e a s e l o v e c o u n t ( r e c e i v e r ) ; 16 } 17 } 18 } In the stories below, user M is a freshman who is competing for the title of most-loved user and hence is very serious about his love counts. (1) (5%) When running the code above, M complains that he should have gotten three love messages from three different junior users J1, J2, J3. Nevertheless, M s love count only shows 1. Is it possible? If so, give an example of three concurrent calls to M.increase love count() that results in such a behavior. If not, state your arguments to M. (2) (5%) A programmer in your team tried to respond to M s complaint by adding the keyword synchronized to the declaration of the increase love count method in POOLoveThread. One day later, M comes back and has the same complaint again. Is it possible? Why or why not? (3) (5%) Another programmer in your team tried to respond to M s complaint by adding the keyword synchronized to the declaration of the increase love count method in POOUser. One day later, M comes back and has the same complaint again. Is it possible? Why or why not? (4) (5%) Assume that the code of POOUser cannot be changed. How would you change the code of POOLoveThread to resolve the complaint of M? 5 of 5